Process for improving the regulating behavior of an anti-lock systems

ABSTRACT

To improve the control behavior of an anti-lock control system or brake slip control system, more particularly, to improve the steerability of a vehicle and its drivability when braking during cornering is performed, criteria for cornering identification and for determining the direction of cornering are derived from the wheel slip of the individual wheels. When cornering is identified, the normal control mode which is intended for straight travel and is configured for an individual control of all wheels or an individual control of the front wheels in conjunction with a select-low control of the rear wheels is changed to a cornering control mode. In the cornering control mode, the average pressure level of the curve-inward front wheel is decreased by a predetermined value and the average pressure level of the curve-outward front wheel is increased by a predetermined value. A corresponding variation of the average pressure level on the rear wheels is also possible in vehicles having a select-low control and valve pairs on the individual wheels.

BACKGROUND OF THE INVENTION

The present invention relates to a method of improving the controlbehavior of an anti-lock control system, more particularly, thesteerability of a vehicle and its drivability during cornering, whereinthe rotational behavior of the vehicle wheels is measured and evaluatedto determine a vehicle reference speed which is used as a referencevalue for ascertaining the wheel slip and for braking pressuremodulation, and wherein criteria for cornering identification and foridentification of the direction of cornering are derived from the wheelslip of the individual wheels.

German patent No. 34 13 738 discloses an anti-lock control system (ABS)including a cornering identification circuit which is also based onmeasuring the wheel slip. For cornering identification, the slip valueson the wheels of one vehicle side are added and compared with the slipsum of the wheels of the other vehicle side. A cornering identificationsignal is produced as soon as the difference of the slip sums exceeds apredetermined limit value. When cornering is identified, selectioncriteria, such as "select-low" or "select-high", by which the pressurevariation in the individual braking pressure control channels of thebrake system is controlled, and limit values for activation of theselection criteria are varied. The objective is to adapt the control tothe different conditions during straight travel and cornering.

The older German patent application No. 21 19 590 teaches producing acornering identification signal by means of a transverse accelerationmeasuring device, for example, a mercury switch.

The object of the present invention is also to provide an improvedcontrol of an anti-lock control system (ABS) or a brake slip controlsystem by adapting the control to the different conditions duringstraight travel and cornering. A special objective is to improve thesteerability of the vehicle and its driving stability during cornering.

SUMMARY OF THE INVENTION

It has been found that this object can be achieved by a method of thetype described hereinabove, the special features of which involve that,when cornering is identified, the normal control mode which is intendedfor straight travel and is configured for an individual control of allwheels or an individual control of the front wheels in conjunction witha select-low control of the rear wheels is changed to a corneringcontrol mode, wherein in comparison to the normal control mode, theaverage pressure level of the curve-inward front wheel is decreased by apredetermined value and the average pressure level of the curve-outwardfront wheel is increased by a predetermined value. In general (eventhough not in all cases), the amount of decrease on the curve-inwardwheel should correspond to the amount of increase on the second wheel ofthis axle.

The concept underlying the present invention is that the desiredimprovement of the control behavior which equally ensures enhancedsteerability and driving stability and a high braking effect, namely theattaining of a minimum short stopping distance, can be achieved bytaking into account the interaction of the longitudinal and corneringforces on all wheels. Taking into account the longitudinal and corneringforces for each individual wheel is not successful. Instead, accordingto the present invention, influencing the longitudinal and corneringforces on the wheels is utilized to develop an understeering yawingtorque which counteracts oversteering of the vehicle caused bycornering. Oversteering is due to the fact that the centrifugalforce--due to an insufficient axle load or due to drive effects andbrake slip--cannot be supported by the sum of all lateral forces.Therefore, according to the present invention, a special control mode isactivated when cornering is identified to correct the average pressurelevel on the curve-inward and the curve-outward wheels (or only of thefront wheels) in the described fashion.

In a preferred aspect of the method of the present invention, the amountof decrease of the average pressure level on the curve-inward wheeland/or the amount of increase thereof on the curve-outward front wheelis varied or predetermined as a function of the rotational speeddifference or the slip difference of the rear wheels.

In a vehicle with a select-low control on the rear axle, appropriately,the average pressure level is decreased on the curve-inward rear wheeland increased on the curve-outward rear wheel in the cornering controlmode. In this case, the rear wheels as well as the front wheelscontribute to developing a yawing torque which compensates oversteering.

In a particularly favorable method of cornering identification, thewheel slip values of the individual wheels are filtered and the filteredvalues are compared, and switch-over to the cornering control mode iseffected when simultaneously the filtered wheel slip on the two frontwheels is in excess of a predetermined front-wheel-related maximum slipvalue that is responsive to the instantaneous vehicle speed or vehiclereference speed, and the filtered wheel slip of one rear wheel is abovea predetermined maximum slip value that is responsive to the vehicle(reference) speed, and the filtered wheel slip of the other rear wheelis below a predetermined minimum slip value that is responsive to thevehicle reference speed, and the rear wheel having the lower slip valueor the higher speed, respectively, is assessed as the curve-outward rearwheel to determine the direction of cornering.

When programmed circuitries, such as microprocessors, microcomputers, orlike elements, and filters with a low-pass characteristic are used,favorably, the filtered wheel slip signals are produced according to therelation

    fws.sub.neu =fws.sub.alt +(fws.sub.alt -fws.sub.neu)/T

and `fws_(neu) ` refers to the filtered wheel slip which was the lastone ascertained in the working cycle and `fws_(alt) ` refers to thepreviously ascertained filtered wheel slip. `T` implies a predeterminedtime constant which ranges in the order between 30 and 200 msec, moreparticularly between 50 and 100 msec.

Further, according to the present invention, the difference of thefiltered slip values of the rear wheels is determined in the corneringcontrol mode, and the control threshold on the curve-inward front wheelwhich defines the average pressure level is increased according to therelation

    ΔRS=k.sub.1 *Δfws.sub.HA +k.sub.2 1/a.sub.FZ

and decreased on the curve-outward wheel. `ΔRS` implies the amount ofthe threshold variation, `Δfws_(HA) ` refers to the difference of theslip values on the rear wheels, `a_(FZ) ` implies the vehicleacceleration, and `k₁, k₂ ` are constants.

The method of the present invention takes effect on the slip control inthe partial braking range, namely in braking operations below thewheel-lock threshold, and on the pressure variation in anti-lock controlbraking operations. The constants k₁, k₂ in the above-mentionedrelation, which applies to the control thresholds, adopt differentvalues in the partial braking range and in the ABS range. Suitably, theoptimum values for k₁, k₂ are determined empirically.

Further features, advantages and possible applications of the presentinvention can be seen in the following description of embodiments,making reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 is a schematically simplified block diagram view of the mainelectronic components of a circuitry for implementing the method of thepresent invention.

FIG. 2 is a flow chart illustrating the operation of the circuit of FIG.1 or the program run when a programmed circuit is used.

DETAILED DESCRIPTION OF THE DRAWINGS

The operating principle of a circuitry for anti-lock or brake slipcontrol is represented in FIG. 1. The rotational behavior of theindividual vehicle wheels is measured by measuring data emitters ortoothed discs 1 to 4 co-rotating with the vehicle wheels and bytransducers or wheel sensors S1 to S4. An alternating voltage isgenerated in the present embodiment having a frequency which correspondsto the wheel speed.

The wheel speed signals v₁ to v₄ are produced by conditioning the thesensor signals in a circuit 5. A vehicle reference speed V_(Ref) isproduced in a known fashion in a combining circuit 6 by logicallycombining the output signals of circuit 5, i.e. the speed signals v₁ tov₄. Among others, the vehicle reference speed is used as a referencevalue to determine the wheel slip λ₁ to λ₄ of the individual vehiclewheels and, thus, as a reference value for braking pressure modulation.

A circuit block 7 includes the individual circuits for determining thewheel slip λ₁ . . . λ₄ by comparing the vehicle reference speed with therespective wheel speed v₁ to v₄. The wheel slip is the result of thedifference

    λ.sub.1 =v.sub.Ref -v.sub.i

    and i=1 . . . 4.

When a programmed circuit, for example, a microcomputer, is used, block7 represents the program steps to calculate the slip λ_(i) (i=1 . . . 4)of the individual vehicle wheels.

Each slip signal passes in a circuit 8 through a filter with a low-passcharacteristic having a filter time constant in the order between 50 and200 msec, for example, 70 msec. A filtered wheel slip signal fws_(i) isproduced. Generally, the relation

    fws.sub.i =λ.sub.i (1-e.sup.t/T),

    with =1 . . . 4

applies to the filtered signal fws_(i). `T` is the time constant of thefilter.

When a digital filter is used, according to the present invention, thefiltered wheel slip signal fws_(i) is calculated according to therelation

    fws.sub.neu =fws.sub.alt +(fws.sub.alt -fws.sub.neu)/T

Of course, the filtered wheel slip signal is ascertained individuallyfor each wheel.

The filtered wheel slip values fws_(i) are sent to an evaluating circuit9 where they are analyzed and processed. Cornering identification withrespect to qualitative and quantitative criteria is performed in circuit9. The direction of cornering is also determined by evaluating andlogically combining the slip signals and slip difference signals. Theresult of this slip assessment by means of circuit 9 is sent through anadditional circuit 10, i.e. a circuit in addition to the proper standardABS control logic 11, to adjust the anti-lock or slip control to thespecial conditions caused by cornering.

The output of the control logic 11 (by way of conditioning andevaluating circuits not shown) finally leads to the actuators ormodulators 12 by which the braking pressure of a brake system isinfluenced in the necessary manner. Generally, up-to-date antilocksystems use electromagnetically operable hydraulic valves, for example,multi-way valves, for the modulation or control of the braking pressurein the individual wheel brakes of a hydraulic brake system.

The proper ABS control logic is also based on the evaluation of theconditioned wheel speed signals v₁ to v₄ in consideration of the vehiclereference speed v_(Ref).

The described operations of the circuitry of FIG. 1 can be achieved byprogrammed circuits, such as microcomputers, etc., as has been explainedhereinabove. Controller technology of this type is even preferrednowadays. The embodiment of FIG. 2 shows a section of a flow chart or aprogram plan illustrating the combining of individual steps of themethod of the present invention. After the `START` (represented byreference numeral 13) of the program section described, it is initiallyestablished (represented by a rhombus 14) whether an ABS mode prevailsat the moment, or whether a partial braking operation (ABS-mode: no) isperformed at the moment.

In the ABS mode, the further program run depends on the decision (at 15)whether cornering identification prevails. If yes, in the embodiment ofFIG. 2, the slip difference or speed difference of the rear wheels iscalculated by a program element 16 according to the relation

    Δfws.sub.HA =|fws.sub.HL -fws.sub.HR |

Subsequently, in a program element 17, the control thresholds on thecurve-outward front wheel are increased and the threshold on thecurve-inward front wheel is decreased according to the relation

    ΔRS=k.sub.1 *Δfws.sub.HA +k.sub.2 1/a.sub.FZ

ΔRS designates the variation of the control threshold, a_(FZ) refers tothe vehicle acceleration and k₁, k₂ are factors. The pressure level isreduced on the curve-inward front wheel and increased on thecurve-outward front wheel by this threshold increase or decrease,respectively. The amount of the decrease or increase is predetermined bythe rotational speed difference or slip difference of the rear wheels,ascertained by program element 16.

The method of the present invention also takes effect on a partialbraking operation, namely a normal braking operation, where wheel locktendencies or wheel instabilities are absent.

When the system is not in an ABS control mode, the program run leads tothe right in the branch point 14. When the vehicle drives around a bend,and this is identified (at 18) by the control, again the slip differenceΔfws_(HA) of the rear wheels is determined by way of program element 19.The slip difference contains an information about the transverseacceleration of the vehicle.

When a braking operation exists, i.e. a partial braking operation, andthe corresponding question (at 20) is affirmed, the control thresholdson the front wheels are varied also in this situation. Exactly as in theabove-described ABS mode, control threshold variations ΔRS' aredetermined in the program element 21 according to the relation

    ΔRS'=k.sub.1 '*Δfws.sub.HA +k.sub.2 1/a.sub.FZ

However, in this case, namely outside an ABS control operation, othervalues apply to the constants k₁ ', k₂ ' than in the ABS mode.

After termination of the threshold increase, the "No" outputs of thedecision points 15, 18, and 20, exactly as the output signals, lead backto the start of the program loop illustrated by way of the programelements 17 and 21. Thus, the polls and actions described are repeatedin the working cycle of the controller.

The method of the present invention permits a simple intervention tostabilize the vehicle during cornering or when braking is effectedduring cornering, in the ABS mode and in partial braking operations. Inthe presence of criteria for cornering identification, a stabilizingyawing torque is generated by variation of the control thresholds ordecrease of the average pressure level on the curve-inward wheels or onthe curve-inward front wheel and increase of the pressure level on thecurve-outward wheels (front wheel). Because the braking pressure on thecurve-outward wheel or on the curve-outward wheels which are subjectedto higher stress during cornering is increased for this purpose (i.e.,the braking pressure variation tendency is "correct") the braking effectis increased and the stopping distance is reduced in addition to thestability benefit. By no means is it necessary to put up with a longerstopping distance for gaining in stability.

The cornering situation which causes transverse accelerations isachieved by logically combining and evaluating the slip values on eachindividual wheel according to the present invention. The need forexpensive transverse acceleration sensors and like elements for thequalitative and quantitative cornering identification is eliminated.

In a vehicle with rear-wheel drive, black-and-white brake circuitallotment and a three-channel ABS system, generally, only one valve pair(inlet and outlet valve) is provided for the control of the rear-axlebraking pressure. According to the present invention, only the brakingpressure in the front wheels or the average braking pressure level inthe front-wheel brakes is varied in the described fashion in comparisonto conventional ABS systems in this case. The curve-inward front wheelis controlled more precisely, i.e., the average pressure level isdecreased, and the control on the curve-outward front wheel becomes lessprecise.

The extent of the decrease and increase results from the difference ofthe rear-wheel speeds, as has been explained hereinabove. The greaterthe difference, the greater the yawing torque caused by the method ofthe present invention must be in order to achieve the stabilizingeffect, or in order to compensate for oversteering of the vehicle whichis due to cornering.

Different amounts of longitudinal forces on the front axle are caused bythe braking pressure modulation described. In addition to the increasedlongitudinal force, the curve-outward wheel also has a reduced corneringforce. The yawing torque (torque about the vertical axis) which isgenerated by these two force components counteracts oversteering of thevehicle and, thus, has a stabilizing effect.

In front-wheel driven vehicles having a diagonal brake circuit allotmentand a three-channel ABS system, the rear-wheel brakes are connected byway of separate valve pairs. In this case, the braking pressurevariation according to the method of the present invention can also beextended to the rear wheels so that the front wheels and the rear wheelscontribute to stabilizing the vehicle during cornering or tocompensating its oversteering behavior. This improvement is preferablyachieved because the generally provided control of the rear-axlepressure according to the select-low criterion is removed at least inpart, and the average pressure level is decreased on the curve-inwardrear wheel and increased on the curve-outward rear wheel.

In vehicles with a front-wheel drive and a diagonal brake circuitallotment, the possibilities of a stabilizing intervention by variationof the pressure level in the front-wheel brakes are slightly reduced dueto drive effects. In these vehicles, the described variation of thepressure level in the rear-wheel brakes with a view to stabilizing thevehicle during cornering or compensating the oversteering tendencybecomes more important.

We claim:
 1. A method of improving the control behavior of an anti-lockcontrol system for a four-wheeled vehicle during cornering, wherein therotational behavior of the vehicle wheels is measured and evaluated todetermine a vehicle reference speed which is used as a reference valuefor ascertaining a wheel slip as well as for braking pressuremodulation, and wherein criteria for cornering identification and foridentification of the direction of cornering are derived from the wheelslip of the individual wheels, wherein, when cornering is identified, anormal control mode--which is intended for straight travel and isconfigured for an individual control of multiple wheels--is changed to acornering control mode, wherein in comparison to the normal controlmode, an average braking pressure level of the curve-inward front wheelis decreased by a predetermined value and an average braking pressurelevel of the curve-outward front wheel is increased by a predeterminedvalue wherein, in a vehicle with a select-low control on the rearwheels, the average pressure level is decreased on the curve-inward rearwheel and increased on the curve-outward rear wheel in the corneringcontrol mode.
 2. The method as claimed in claim 1,wherein the amount ofvariation of at least one of the average braking pressure levels is afunction of the rotational speed difference of the rear wheels.
 3. Themethod as claimed in claim 1 wherein, when programmed circuitries andfilters with a low-pass characteristic are used, the filtered wheel slipsignals are produced according to the relation

    fws.sub.neu =fws.sub.alt +(fws.sub.alt -fws.sub.neu)/T

and `fws_(neu) ` refers to the filtered wheel slip which was the lastone ascertained, and "fws_(alt) ` refers to the previously ascertainedfiltered wheel slip, and `T` implies a predetermined time constant whichranges in the order between 30 and 200 msec.
 4. The method as claimed inclaim 3, wherein the time constant ranges in the order between 50 and100 msec.
 5. A method of improving the control behavior of an anti-lockcontrol system for a four-wheeled vehicle during cornering, wherein therotational behavior of the vehicle wheels is measured and evaluated todetermine a vehicle reference speed which is used as a reference valuefor ascertaining a wheel slip as well as for braking pressuremodulation, and wherein criteria for cornering identification and foridentification of the direction of cornering are derived from the wheelslip of the individual wheels, wherein, when cornering is identified, anormal control mode--which is intended for straight travel and isconfigured for an individual control of multiple wheels--is changed to acornering control mode, wherein in comparison to the normal controlmode, an average braking pressure level of the curve-inward front wheelis decreased by a predetermined value and an average braking pressurelevel of the curve-outward front wheel is increased by a predeterminedvalue wherein, for cornering identification, the wheel slip values ofthe individual wheels are filtered and the filtered values are compared,and wherein switch-over to the cornering control mode is effected whensimultaneously the filtered wheel slip on the two front wheels is inexcess of a predetermined front-wheel-related maximum slip value that isresponsive to a vehicle reference speed, the filtered wheel slip of onerear wheel is above a predetermined rear-wheel-related maximum slipvalue that is responsive to the vehicle reference speed, and thefiltered wheel slip of the other rear wheel is below a predeterminedrear-wheel-related minimum slip value that is responsive to the vehiclereference speed, and wherein the rear wheel having the higher speed isassessed as the curve-outward rear wheel to determine the direction ofcornering.
 6. The method as claimed in claim 5, wherein a value in theorder between 3 and 10% of the vehicle reference speed is predeterminedfor the front-wheel-related maximum slip value, a value in the orderbetween 2 and 5% of the vehicle reference speed is predetermined for therear-wheel-related maximum slip value, and a value in the order between0.5 and 2% of the vehicle reference speed is predetermined for therear-wheel-related minimum slip value.
 7. A method of improving thecontrol behavior of an anti-lock control system for a four-wheeledvehicle during cornering, wherein the rotational behavior of the vehiclewheels is measured and evaluated to determine a vehicle reference speedwhich is used as a reference value for ascertaining a wheel slip as wellas for braking pressure modulation, and wherein criteria for corneringidentification and for identification of the direction of cornering arederived from the wheel slip of the individual wheels, wherein, whencornering is identified, a normal control mode--which is intended forstraight travel and is configured for an individual control of multiplewheels--is changed to a cornering control mode, wherein in comparison tothe normal control mode, an average braking pressure level of thecurve-inward front wheel is decreased by a predetermined value and anaverage braking pressure level of the curve-outward front wheel isincreased by a predetermined value wherein a difference of filtered slipvalues of the rear wheels is determined in the cornering control mode,and a control threshold on the curve-inward front wheel, which definesan average pressure level, is increase according to the relation

    ΔRS=k.sub.1 *Δfws.sub.HA +k.sub.2 1/a.sub.fz

and decreased on the curve-outward wheel, and `ΔRS` implies the amountof the threshold variation, `Δfws_(HA) ` refers to the difference of theslip values on the rear wheels, `a_(fz) ` implies the vehicleacceleration, and `k₁ k₂ ` are constants.
 8. The method as claimed inclaim 7, wherein, during brake operations without anti-lock control, theconstants k₁ k₂ adopt different predetermined values versus brakeoperations with anti-lock control.